Introduce a 'PackPrivate' mechanism for private project reference packing

[NinoFloris]

Context:
NuGet/Home#3891

What I ideally want to see is a mechanism that:

• Allows dependencies to flow transitively in builds (so I don't need to redefine references in test projects and so on, today PrivateAssets=All disables transitive flow).
• Disables transforming project references into nuget package dependencies (PrivateAssets=All also affects this part today).
• Packs the outputs of the project into the package without needing to include all kinds of target magic (see Feature : Allow project reference DLLs to be added to the parent nupkg for pack target like IncludeReferencedProjects in nuget.exe NuGet/Home#3891 (comment)).

As I understand it PrivateAssets is overloaded and combines local reference semantics with packaging behaviors, this is problematic. Additionally, due to the complexity of the restore phase (assets.json etc) it seems impossible to create these kinds of PackPrivate features outside of the sdk while achieving good integration. I just don't believe the hooks are there to do so.

Related:

• https://github.com/search?q=TargetsForTfmSpecificContentInPackage+WithMetadataValue%28%27PrivateAssets%27%2C+%27All%27%29&type=code&p=1
• Feature : Allow project reference DLLs to be added to the parent nupkg for pack target like IncludeReferencedProjects in nuget.exe NuGet/Home#3891